In the manufacture of semiconductor devices it is advisable that such components be tested at the wafer level to evaluate their functionality. The process in which die on a wafer are tested is commonly referred to as "wafer sort." Testing and determining design flaws at the die level offers several advantages. First, it allows designers to evaluate the functionality of new devices during development. Increasing packaging costs also make wafer sorting a viable cost saver, in that the reliability of each die on a wafer may be tested before incurring the high costs of packaging.
Wafer sorting typically involves the use of probing technology wherein a probe card containing probe features engages the bond pads on a die so as to connect the pads to a tester. FIGS. 1A, 1B and 1C illustrates a typical testing apparatus including a tester 10, test head 11, and handler 12, that is used to test the performance of a die on a wafer. As illustrated, probe card 14 sits below and in contact with test head 1 1. During testing, the handler supports the wafer on platform (chuck) 16 and positions the wafer so as to precisely align the bond pads of a die to be tested with the probe features on the probe card. Chuck 16 is connected to a staging device 18 by rods 17. The staging device 18 typically positions the chuck along an x-y plane by moving along a stage floor 13 on a ball screw stage assembly. Staging device 18 may also position the chuck by floating above the stage floor on a magnetic air bearing. Chuck 16 typically comprises a vacuum chuck wherein the wafer being tested is held in position by drawing a vacuum within a plurality of interconnecting channels 19 that are formed within the surface of the chuck. Once aligned, chuck 16 is raised via rods 17 such that the bond pads of the die are forced against the probe features on the probe card.
All categories of probing utilize some form of "scrub" at the touch down phase of a probe feature to a bond pad. Scrub applies to probed aluminum or lead, where the probe features on a probe card pierce (scrub) the layer of oxide, a nonconductive film that grows quickly on exposed aluminum and lead. Generally, scrub applies to any nonconductive layer that produces a barrier between the test probes of a probe card and the base metal of a bond pad. The purpose of the scrub is to break through the non-conductive layer on the bond pads in order to establish a good electrical contact between the probe features and the base metal of the bond pads. Scrub occurs when the handler forces the wafer, and, subsequently, the bond pads of a die, against the probe features on the probe card causing the probe features to deflect. The scrub is generated by a small horizontal movement of each probe feature across the surface of each corresponding bond pad as the probe features deflect. As the probe features move across the bond pads they penetrate the nonconductive oxide layer thereby establishing a good electrical contact between the probe features and the bond pads. This type of scrub is referred to as "passive" scrub. Typically, the amount of deflection of the probe features, and, hence, the amount of scrub achieved, is proportional to the force applied by the movement of the wafer against the probe card features. The additional movement of a wafer toward a probe card after initial contact with a probe feature is known as "overdrive."
Probe cards presently available are of the passive scrub type. The cantilever tungsten needle probe card 20, as illustrated in FIG. 2, is one example. As shown in FIG. 2, probe card 20 possesses a fiberglass epoxy-base printed circuit board 21 with tungsten needles 22 extending out from the probe card and held in position by an epoxy ring 28. Each needle contains a tip (probe feature) 23 for making contact with the bond pads of a die. As previously discussed, the amount of scrub achieved on the surface of a bond pad is proportional to the force applied by the movement of the wafer against the probe card features. A tungsten needle probe card, as illustrated in FIG. 2, typically requires overdrive levels of 0.002 to 0.004 inches to achieve good electrical contact at the bond pads.
There are a number of problems associated with the passive scrub cantilever needle probe card. First, the high overdrive levels required to achieve good electrical contact between the probe features and die bond pads cause the probe features to bend, break and wear more quickly, resulting in increased replacement and repair costs. High overdrive also increases the probability that deep and damaging scrub marks will result making it difficult to bond wires to the die pad. Another problem associated with passive scrub cantilever needle probe card is that it sometimes requires two or more touchdowns per die test to break through the pad oxidation layer. This creates two problems. First, it prolongs the amount of time required to perform a die test. Secondly, it diminishes the effective life of a probe card.
Yet another problem with passive scrub needle cards is that stray particle and oxide buildup often occur at the tip of the probe features. Stray particle and oxide buildup contributes to high contact resistance between the probe feature and bond pad. High contact resistance causes inaccurate voltage levels during device testing due to the voltage produced across the probe tip. This may cause a device to incorrectly fail test resulting in lower test yields.
Membrane probe cards were developed to address some of the problems associated with tungsten needle probe cards. Namely, membrane probe cards were developed to provide a smaller and more uniform scrub along the bond pad surface. FIGS. 3A and 3B illustrate a cross-sectional view of a typical membrane probe card. As shown, probe card 30 possesses a flexible printed circuit (membrane) 31 having spherical contact bumps (probe features) 32. The contact bumps are generally coupled to an epoxy-base printed circuit board 34 that provides structural support for membrane 31 and electrically connects probe card 30 to the test head of a wafer sort tester. Photolithography is used to manufacture contact bumps 32, hence, the probe feature geometries that are achieved are small and precise. Column spring 33 provides both support and resilience to the membrane structure. The spring constant of column spring 33 establishes the amount of force that is required to deflect membrane 31 in order to achieve scrub at the contact bump and die pad interface.
Although smaller and more precise scrub geometries are achievable using membrane type probe cards, there exists two major draw backs associated with their use. First, the amount of scrub that is actually obtained is typically minimal or non-existent. That is, the probe features are often unable to pierce the oxidation layer of the die pad unless the die pad is very clean. Secondly, membrane probe features 32 are prone to clogging and must be cleaned on a relatively frequent basis. Because it is difficult to clean the membrane features without damaging them, the useful life of a membrane probe card is typically much shorter than that of a cantilever needle probe card. For these reasons, the use of membrane probe cards are generally avoided in full-scale manufacturing applications.
Thus, what is needed is a method and apparatus for electrically testing the functionality of wafers that solves the problems associated with current passive scrub techniques.